The present invention relates in general to control systems which include and utilize a programmed digital computer. More particularly, the invention pertains to apparatus by which intelligence is taken into a computer from external devices (such as parameter sensors) and by which intelligence is outputted from the computer to external devices (such as condition-influencing actuators).
In control systems of the type mentioned above, a great number of external sensors are often used to feed information into the computer. The computer determines and sends out commands to a large pluarlity of external actuators and display units. Each external sensor produces a signal which is of one of two types, i.e., either a signal which can vary to take on any value with a predetermined range (often called an analog signal) or a bistate signal which resides at one of two levels (often called an on-off or digital signal). Likewise, each external actuator may be of a type which responds to a variable analog signal or of the type which responds to an on-off digital signal.
For example, in computer based systems for controlling the heating, air conditioning and humidity in the different office regions or zones of a large office building, a temperature sensing device (thermostat) may be located in each individual office, and a humidity sensor might be located in each of several zone ducts through which air is forced by a blower from a central furnace and air conditioner. Some of the thermostats might be of the on-off type in which switch contacts are opened or closed when sensed temperature is above or below the respective set points; others might be of the variable type in which a thermally sensitive resistor exhibits a resistance value that varies with its temperature. The humidistats likewise may be either of the on-off or the variable type. In such building control system, the computer takes in signals from all of the various external sensors and, by rapid iterations through its programmed algorithm of numerical and Boolean calculations, determines which output commands, and the magnitudes thereof, are to be sent to condition-influencing external actuators. Typically, the computer may send output command signals to damper-positioning motors so as to adjust the proportions of the total blower-pumped air sent through the several respective zone ducts, send signals which change the speed of a blower motor, and/or send signals to change the opening of a furnace fuel valve, as well as signals which merely open or close certain valves or turn indicator lamps on or off.
Accordingly, there is a need to interface a whole host of external input devices (e.g. sensors) and external output devices (e.g. actuators) to a central computer which takes intelligence in from the former and sends back intelligence (commands) to the latter. Each external device is either of the variable (analog) or on-off (digital) type; each external device either sends input signals to, or takes output signals from, the computer. While flow of information from or to the several external devices can be achieved by rapid time sharing and multiplexing accomplished through repeated passes (iterations) through the computer's programmed algorithm, there must be a physical connection and a signal path provided for each of the many external devices. It is not unusual for an entire system to employ as many as five hundred external devices.
The interface hardware for such systems thus includes a large number of terminal points to which respective ones of the external devices may be connected. Early in the art, each terminal point was dedicated to coact with one of the four classes of external devices, i.e., (I) an analog output device, (II) a bistate output device, (III) an analog input device, or (IV) a bistate input device. Once the hardware had been manufactured, its lack of flexibility made installation and changes in the field difficult. Standard computer/interface hardware made with sixteen terminal points for each of the four classes, for example, could not be hooked up to accommodate, say twenty-four devices of Class I, twenty-four devices of Class II, plus ten devices of Class III and twelve devices of Class IV. If the control system for a building were modified after the computer and interface hardware had been delivered to the site, that hardware would not always "fit" to a modified mix of the external devices occasioned by last-minute design changes in the building or its heating system.
To overcome such inflexibility, the interface hardware was improved to provide "universal" terminal point interface circuits. That is, the circuitry between the computer and a given terminal point included a digital-to-analog converter, a voltage follower, and a solid state (FET) gate for outputting a dc. voltage to an external device of Class I or II, together with an analog-to-digital converter between the terminal point and the computer for inputting to the computer a digital representation of the voltage produced by an external device of Class III or IV. When, and if, a Class I or II external device was in fact connected to that given terminal point, a hard wiring connection was made in the interface circuit board so that a gating signal was permanently applied to enable the FET gate at all times when the computer was "powered up". When, and if, an external device of Class III or IV was in fact connected to that given terminal point, the hard wiring was omitted so that the gate was never enabled; dc. voltage from the external device could thus be taken into the computer via an analog-to-digital converter (ADC), and that input voltage was isolated from the output of the voltage follower.
Such prior "universal" input/output interface hardware was, however, expensive. Each terminal point circuit necessarily required an FET gate costing on the order of $1.50 to $2.00; for sixty-four terminal points, the interface circuit boards were thus burdened with a gate cost on the order of $112 in addition to the cost of providing and wiring in the components to send enabling signals to those gates. The FET gates also had the disadvantage of creating inaccuracies in the analog output voltage in relation to the input voltage fed to the voltage follower.